1. Field of the Invention
The present invention relates to a solid-state image sensing device, and more particularly to a solid-state image sensing device having a photoelectric conversion characteristic comprised of two or more different characteristic regions, a method for driving the solid-state image sensing device, and an image sensing system incorporated with the solid-state image sensing device.
2. Description of the Related Art
In recent years, in an image sensing apparatus such as a digital camera, as high-quality performance is demanded, a technology of expanding a luminance range i.e. a dynamic range of a subject image to be captured by a solid-state image sensing device i.e. an image sensor is required. In expanding the dynamic range, there is known a solid-state image sensing device (called as a “linear-logarithmic sensor”) having a photoelectric conversion characteristic comprised of a linear characteristic 901 and a logarithmic characteristic 902 (also, called as “linear/logarithmic characteristics”) as shown in e.g. FIG. 18 (see e.g. Japanese Unexamined Patent Publication No. 2006-50544 (D1)). The linear-logarithmic sensor is capable of performing linear output and logarithmic output. Hereinafter, an image having linear and logarithmic characteristics captured by the linear-logarithmic sensor may be called as a “linear-logarithmic image”. The point indicated by the reference numeral 903 in FIG. 18 is a switching point i.e. the inflection point 903 between the linear characteristic and the logarithmic characteristic. With use of the linear-logarithmic sensor, the position of the inflection point 903 can be flexibly changed. By changing the inflection point 903, a ratio of the linear characteristic 901 and the logarithmic characteristic 902 is changed to thereby realize a photoelectric conversion characteristic having a larger ratio in linear characteristic region, or a photoelectric conversion characteristic having a larger ratio in logarithmic characteristic region.
In the aforementioned solid-state image sensing device, for instance, increase of a gain by a column amplifier in a column circuit of the solid-state image sensing device in order to secure gradation i.e. contrast in a dark portion of a subject image may cause an increase in gain with respect to both of the linear characteristic 901 and the logarithmic characteristic 902. In the example shown in FIG. 18, the photoelectric conversion characteristic is comprised of the linear characteristic 901 and the logarithmic characteristic 902 may turn into a photoelectric conversion characteristic comprised of a linear characteristic 901′ and a logarithmic characteristic 902′. Since the output saturation level of a solid-state image sensing device is restricted, a considerable part of the linear characteristic 902′ may exceed the output saturation level. As a result, the dynamic range may be narrowed from a range indicated by the arrows 904 before the gain increase to a range indicated by the arrows 905 after the gain increase.
In the case where the solid-state image sensing device disclosed in D1 is used, it is possible to switch over the output from each of pixels between the linear output and the logarithmic output with respect to each of frame images by a method for driving each of the pixels. However, in this arrangement, it is necessary to use characteristic images of two frames i.e. one frame corresponding to a linear characteristic image and one frame corresponding to a logarithmic characteristic image to reproduce one frame image having both of the linear characteristic and the logarithmic characteristic as a final image. As a result, the frame rate for reproducing an image may be lowered to half of the frame rate at which an image is reproduced by outputting either one of the two characteristic images. In other words, although the above arrangement is capable of producing an image having a wide dynamic range, the frame rate may be lowered.